1. Field of the Invention
This invention relates to a film forming apparatus appropriate for forming a monomolecular film or a monomolecular built-up film (hereinafter referred to as "LB film"), and particularly to the film forming apparatus which can reliably obtain an LB film in which molecular layers are arranged in a predetermined order.
2. Description of the Prior Art
Until recently, inorganic substances, which are relatively easy to handle, have been exclusively used in semiconductor and optics technology. One reason might be that the technology of organic chemistry was far behind that of inorganic chemistry.
However, the technology in organic chemistry has strikingly progressed in recent years and, moreover, the development of materials composed of inorganic substances seems to have slowed down. Therefore, a functional organic material surpassing the available inorganic substances has been desired. The advantages of the organic material are low price, easy production, excellent functional properties and so on. Further, organic materials have been developed recently which have superior heat-resistance and mechanical strength to those in the prior arts. With such a technical background, several research organizations presented recently he idea of utilizing an organic thin film in place of the conventional inorganic thin film for a part or the whole of the functional portion (principally a thin layer portion) of an integrated circuit device including a logical element, a memory device, a photoelectric transducing device, etc. and of optical devices including a microlens array, an optical wavepath, etc. They also presented the idea of preparing a molecular electron device by giving a function of a logic element, memory device or the like to an organic molecule, or of preparing a logic element comprised of a bio-material (e.g., bio-chips).
The organic thin film, which is the main constituent element of such devices, is formed by a monomolecular built-up method. The monomolecular built-up method (or Langmuir-Blodgett method) is a method which comprises forming a monomolecular film which is spread in an orderly manner on a water surface by utilizing a hydrophilic or hydrophobic property of a molecule having a hydrophilic or hydrophobic group, respectively, and transferring the monomolecular film onto a substrate surface, and it is possible to form the monomolecular film or the monomolecular built-up film in which the monomolecular films are built up on the substrate.
Until recently, referring to FIG. 7, a frame in the apparatus used in this method is horizontally placed to divide a water surface 3 in the inside of a shallow and broad rectangular water tank 1. The frame 2 acts as a two-dimensional cylinder and the rectangular floating element 4 is floated on the water surface in the inside of the frame 2. The floating element 4 has a width a little smaller than the inner width of the frame 2 and can smoothly move forward or backward as a two-dimensional piston. In order to permit floating element 4 to move from side to side, the floating element 4 is connected through a wire 5 with the winding device 6 using a motor or the like.
During formation of the monomolecular film, the constituent of the film dissolved in a volatile solvent such as benzene, chloroform and the like is added dropwise to the water surface 3. The monomolecular film remains on the water surface 3 after evaporation of the solvent, showing a two-dimensional behavior on the water surface 3. When the surface density of molecules in low, the monomolecular film is called a two-dimensional gaseous film. The floating element 4 is slowly moved forward, i.e. toward the right side in the drawing, to reduce the extension of water surface 3 on which a monomolecular film is spread, resulting in an increase in surface density. Then, the interaction between the molecules is intensified and the gaseous film is converted to a two-dimensional solid film through a two-demensional liquid film. In the solid film, the molecules are arranged in uniform orientation, and ultimately the solid film has a high orderliness and a uniform ultrathinness as required for a material constituting a semiconductor.
As a method for transferring the monomolecular film onto the surface of a substrate 7 from the water surface 3, there may be used a vertical dipping method by which the monomolecular film can be transferred by moving up and down the substrate 7 supported by a substrate holder 8 in a vertical direction 9, while applying a constant surface pressure suitable for a build-up operation to the monomolecular film on the water surface 3.
In the case of the vertical dipping method, three kinds of film structure, as shown in FIG. 8, can be obtained. That is, there are a type X where the monomolecular film 10 can adhere only when the substrate 7 is dipped, as shown in FIG. 8(a), a type Y where the monomolecular film 10 can adhere both when the substrate 7 is dipped and when it is withdrawn, as shown in FIG. 8(b), and a type Z where the monomolecular film 10 can adhere only when the substrate 7 is withdrawn, as shown in FIG. 8(c). In the molecule in FIG. 8, 11 is a hydrophilic moiety and 12 is a hydrophobic moiety.
When, for example, a heteroconstruction built-up film of the type Y (constituent molecules of the monomolecular films being different in the direction of built-up) as the built-up film in FIG. 3 is produced by using an apparatus of FIG. 7 (cf. FIG. 8(b)), that is, when a heterojunction is formed between hydrophilic groups 11a of film A and hydrophilic groups 11b of film B, a substrate is moved up and down to attach film A1, and film A2, and the monomolecular film A on the water surface is discarded while the substrate is kept in water. After cleaning the water surface, a monomolecular film B is formed on the water surface, and the substrate is moved up and down to attach film B, and film B2 followed by further attaching film A3 according to the same procedure.
However, upon the exchange of one film for another film, it takes a long time to clean the water surface, and the greater the number of films, the longer the time required for cleaning.
Also, during the cleaning, the water surface is ruffled and the water level varies, and thereby the build-up film on the substrate in the vicinity of the water surface is not orderly arranged and can not be used.
The conventional apparatus as described above is too time-consuming and requires much labor for the formation of the heteroconstruction built-up film, that is, a film constituted of different kinds of molecular layers.
In addition, a film which can still be used should be disadvantageously discarded.